Magnetic-tape drop-out compensator



March 28, 1961 w. R. JOHNSON 24,956

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United States Patent 24,956 MAGNETIC-TAPE-DROPDUT COMPENSATOR Wayne R.Johnson, 'Los Angeles, Calif., assignor to Minnesota Mining andManufacturing Company, St. Paul, Minn., a corporation of DelawareOriginal No. 2,816,162, dated Dec. 10, 1957, Ser. No. 393,844, Nov. 23,1953. Application for reissue Feb. 12, 1959, Ser. No. 792,937

18 Claims. (Cl. 178-66) Matter enclosed in heavy brackets appears in theoriginal patent but forms no part of this reissue specification; matterprinted in italics indicates the additions made by reissue.

This invention relates to the reproduction of television signalsrecorded on a magnetic medium, and particularly to means and methods forcompensation of drop-outs or gaps in the reproduced signals due toimperfections in the recording medium.

The band of frequencies which must be recorded andreproduced in order toprovide a satisfactory television image, under present standards oftransmission, comprises those extending from zero (direct current) tofour million cycles. The upper portions of this band include frequencieswhich are far too high for practical recording by the simple methodswhich are applicable to the relatively narrow sound spectrum. In thesystem for which the present invention is specifically intended theoriginal signal is efiectively broken up into a plurality of differentwave trains which are separately recorded, reproduced, and recombined toreconstitute a signal which closely approximates the original. Thissystem is-described in detail in copending applications of John Mullin,Serial No. 195,612, filed November 14, 1950, (now Patent No. 2,794,066),and of the present inventor, Serial Nos. 272,083, (now Patent No.2,867,685), 272,084 (now Patent No. 2,694,748) and 272,085 (now PatentNo. 2,695,331), filed February 18, 1952. Briefly, in accordance with theinventions described in these copending applications, a carrierfrequency is generated which is a subharmonic of the highest frequencywhich the recording system is intended to reproduce, and which is withinthe recording capabilities of the electromagnetic transducers used forrecording and reproduction. From this carrier frequency pulses aredeveloped having a fundamental frequency equal to that of the carrier.These pulses are supplied to a plurality of circuits in differentphases, the train of pulses supplied to each circuit beingphase-displaced with respect to those in other circuits by successiveincrements so that, considering the circuits as .a whole, pulses occurat uniform intervals, there being a pulse for each picture element whichis to be reproduced. in any one of the circuits, however, occur at Thesepulses, when they occur, are modulated by the instantaneous amplitudetelevision signals, which is then recorded as a plurality ofamplitude-modulated tracks disposed in parallel upon the common medium.In reproduction, each of these tracks is engaged by a separatetransducer head and fed into a separate circuit. The recorded waves arepreferably distorted somewhat from the simple sine form and flattened atthe top so that there is a relatively long portion of the half-cycle ofthe recorded wave when their amplitude is of maximum value. A wave ofthe same frequency as the original carrier is developed, and dividedinto separate trains of similarly phase-displaced pulses. These pulsesare used to sample the flat topped half-cycles developed in the variouschannels and to switch the instantaneous samples thus taken into acommon channel, thus reconstructing the original signal. v

"ice

The method of television transcription thus described produces excellenttelevision images as long as the recording medium is free from defects.Frequently, however, the oxide coating on the tape, which is thepreferred recording medium, has in it either a pin hole or a smallgranule which projects above the surface. In either case the result is adrop out, where the signal from one or more of the transducer headsfails to record or to reproduce. This produces a defect in the image onthe screen; a series of dark or black dots across the field of view.

As has been stated the television signal contains all frequencies fromzero to a maximum of 4 me. The zero frequency or D.C. componentrepresents the average illumination of the picture field, around whichillumination varies to give bright or dark detail. A change of scenewill, of course, vary the magnitude of the D.C. component. Furthermore,where there are differences in average illumination across the fieldthere will be superimposed, upon the D.C. component, frequencycomponents which may vary, at harmonics of the field and linefrequencies, up to the highest frequencies transmitted, the lowfrequencies merging gradually with the high. The total interruption ofthe signal by a drop-out represents a pulse of opposite sign to the D.C.component and superimposed low frequency components which carries theinstantaneous value of the picture current to zero. How many of thetransducer heads such a drop-out affects depends on the nature of theimperfection causing it. A pin hole will probably affect only one orpossibly two heads. A granule may lift the tape away from a number ofheads causing more black dashes, more closely spaced on the screen.Owing to a combination of mechanical and electrical effects a singledrop-out may affect as many as three lines. Drop-outs occur at random;the chance that one will occur in the same part of the picture field intwo successive frames is very small, but they may occur several timesper second in each track, so that they seriously affect the over-allpicture, making it noisy.

The individual transducer heads treat each signal individually, withoutregard to what the other heads are reproducing immediately before andimmediately after the signal from the specific head is sampled. The lowfrequency components, however, affect all of the heads equally. Theeffect of drop-outs is disturbing largely because of their high degreeof contrast with the average level of illumination of the picture areain which they occur. The repetition rate of the television images is sohigh that if the average level of illumination of the screen isunchanged by them there is no effect of loss of detail, for the detailis supplied by preceding and succeeding images.

The primary object of the present invention is to pre- -vent drop-outsin magnetic reproduction of television lmages from producing noticeableeffects [uopn] upon the quality of the images reproduced. In greaterdetail, among the objects of the invention are to provide means andmethods of maintaining lower frequency components of the picture signalsat substantially a constant level, thus preventing drop-outs fromaffecting more than individual picture points; to provide means forreproducing recorded signals which introduces into the reproducer outputthe low frequencies in a manner which is substantially free of drop-outeffects and is not disturbed by either a single pulse or a plurality ofpulses introduced into any one channel or a plurality of channels bydefects in the reproducing medium; to provide .a general method ofdropout suppression which may be applied in a number of alternative waysand which, by introducing additional refinements can accomplish thesuppression of the dropout effect to almost any degree desired where therequirements are more severe and additional complication is warranted;to provide methods of and means for recording low-frequency componentsof television signals which are substantially resistant to drop-outeffects; to provide means whereby low-frequency components, recorded byfrequency modulation instead of amplitude modulation, can be added tothe amplitude-modulated high-frequency components; to provide means forcomparing the lower frequency components as recorded on the individual,amplitude-modulated tracks with those same components as modulated onthe frequency-modulated tracks, and of adding to the outputs of thevarious amplitude modulated channels only enough low frequencycomponents to compensate for the loss thereof due to the drop-outs; andto provide the means and methods of compensating fordrop-outs whereinone or two channels and tracks provide drop-out compensation for all ofthe multiple tracks on which detail is recorded, in contradistinction toprior methods of compensation wherein a plurality of transducer headsand a plurality of tracks have been provided to insure against drop-outsin each individual track.

Considered broadly, in accordance with the present invention thetelevision signal to be recorded and reproduced is amplitude-modulatedupon a plurality of phasedisplaced carriers of the same frequency, ashas already been described, and is recorded on a plurality of paralleltracks on a common medium. In addition the same signal is preferablyfrequency-modulated upon a carrier, the frequency whereof may beapproximately the same as that used to carry the amplitude-modulatedsignals. The frequency-modulated carrier is recorded upon additionalparallel tracks on the same medium. In reproduction, the signalsrecorded on the amplitude-modulated tracks are picked up as has alreadybeen described and the components of these signals representative of thelower frequency components are subtracted from the signals developedfrom the individual tracks. Simultaneously there is developed from aplurality of the tracks a signal representative of the lower-frequencycomponents common to all of the tracks; preferably the last-mentionedsignal is derived from the frequency-modulated tracks above mentioned,but it may, instead, be derived by combining the lower-frequencies fromenough of the amplitude-modulated tracks to insure that it issubstantially drop-out free. This latter signal is added to those fromthe individual tracks, from which the lower frequencies have beenremoved, and thereafter the signals are sampled and the samples combinedto reconstruct the television signal, with the effect that the averageeffective level is restored and a drop-out will present no sharpcontrast with adjacent portions of the image.

The invention will be more clearly understood by reference to theaccompanying drawings and the detailed description thereof which follow.In these drawings:

Fig. 1 is a diagram, largely in block form, illustrating the inventionin a simple form;

Fig. 2 is a block diagram of a modified form of the reproducingequipment for a single channel;

Fig. 3 is a schematic diagram of circuits employed in the equipmentillustrated in Fig. 2;

Fig. 4 is an idealized graph of a reproduced television waveform; and

Fig. 5 is a similar graph of the same waveform, including the effect ofa drop-out in one channel.

Considering first the primary form of the invention illustrated in Fig.1, the drawing shows, in block form, the equipment utilized for bothrecording and reproducing a television signal. Such a signal may bedeveloped directly from a camera chain, in immediate proximity to therecording equipment, or it may be received over a radio channel. In anyevent it is applied to a terminal 1, representing the common input toall the recording channels. It is fed to an encoder 3, wherein it ismodulated upon a plurality of phase-displaced carriers .of the samefrequency, as is described for example, in the copending application ofthe present inventor, Serial No. 272,083 above identified. The encoderthere shown comprises a group of double balanced modulators of the ringtype, one modulator being provided for each channel into which thesignal. is to be divided for recording purposes. As is well known, suchmodulators comprise three conjugate circuits, two inputs and one output.One: of the input circuits of each modulator is connected to the inputterminal 1. The other input circuit is connected to a sectional delayline which is provided with as many taps as there are modulators, thedelay between successive taps being equal and the total delay beingwhere p is the period of the carrier frequency. The output circuits ofthe various modulators are each connected to a separate recording head.As the modulators are of a double balanced type no signal appears in theoutput circuit unless there are signals applied to both circuits. Thedelay line is supplied with sharp pulses of the carrier frequency,positive and negative pulses alternating in' altern-ate half-cycles. Asa pulse reaches the modulators in succession each instantaneously passesa pulse of current whose amplitude is proportional to the instantaneousmagnitude of the signal and whose sign, positive or negative, depends onthe sign of the delay line pulse. In the specific embodiment theredescribed the signal is sampled by pulses derived from a 169.5 kc.oscillator 4 fed into separate channels in which the pulses arephase-displaced by increments. of 18. Pulses are derived from bothpositive and negative halves of the waves, resulting in wave crests,either positive or negative, every 18 electrical degrees of the 169.5kc. carrier. As a result of this arrangement, considering all tenchannels, 3,390,000 samples per second are taken of the televisionsignal. Each channel supplies a recording head 5, which is substantiallyof conventional form, although the gap in the magnetic circuit of therecorder head is preferably somewhat narrower than that used inrecording sound.

The same video signal is fed to a frequency modulator 7, which may be ofany conventional type, and is modulated on a carrier in the same generalrange as the 169.5 kc. frequency, so that it will permit recording andreproduction of frequencies up to about of the maximum picturefrequencies to be reproduced. The frequencymodulated signal is suppliedto a recording head 9' and recorded upon the same magnetic tape inparallel to the tracks produced by the amplitude-modulated signals. Asan added precaution the same signal is supplied to two transducer heads,in parallel to avoid effects of drop-outs on the PM signals.

Signals are recorded, as has been stated, in parallel tracks upon thesame medium, which, in practice, is a plastic tape, coated with finelydivided iron oxide, the tape being symbolized by the line 11 which, inthe diagram, separates the recording and reproducing equipment. Inreproduction this is picked up by transducer heads '5' and 9, which may,in fact, be the same heads as were used for recording or they may beembodied in entirely separate equipment; they must, however, besimilarly arranged so that they will engage tracks in which the phasesare displaced in the same order, in-so-far as the amplitudemodulatedtracks are concerned and the transducer head 9' must engage afrequency-modulated track in order that the resultant signal may beoperated upon properly.

The signals from the amplitude-modulated tracks are fed to individualpreamplifiers 13, each of which is provided with an automaticgain-control circuit. The time constant of the gain control circuits asbuilt into the amplifier should be less than the period of the highestfrequency waves supplied from the FM tracks; otherwise they areconventional.

V The PM signals also are fed to a preamplifier 15 and thence to afrequency demodulator 16 for demodulating the FM signal, derivingthemodulating components therefrom. "The output from .the demodulator isagain divided into two paths. One of these paths feeds through anintegrating or filter circuit comprising a series resistor 17 and ashunt capacitor 19. The integrating circuit has a time constant of theorder of the period of one scanning line, or 63 microseconds. Its effectis to filter from the signal any remaining higher frequency components.The resultant signal is supplied to the automatic gain controls of thepreamplifiers 13 so as to reduce the amplification with increase of thecontrol signal.

The second low-frequency-component path is through a second integratingcircuit comprising a series resistor 21 and shunt condenser 23,substantially identical with that in the first path, or, at least,having substantially the same time constant. From the integratingcircuit there is a connection to the grid of a cathode follower 25,which supplies sufiicient power to restore the low-frequency componentsat the sameamplitude and in proper phase to all the output circuits ofthe preamplifiers 13, through decoupling resistors 29.

It will be recognized that the changes in gain of the preamplifiers 13,which result from the automatic gain control signals constitute, infact, a modulation process. A ready method of controlling the gain isthe one most commonly used in automatic gain controls, i.e., the use ofremote cut-off pentodes, whose amplification varies with the grid bias.The only real difference between this arrangement and the conventionalone used on many radio and television receivers is that the integrationof the signal used to control the gain is derived for all the amplifiersin common from the integrating circuit 17, 19.

Each of the output circuits, containing both high and low frequencycomponents, leads to an individual switch in a decoder 31, which,actuated by a carrier wave derived from one or more of the recordedsound tracks, samples the various signals at the proper epoch of thecarrier cycle, effectively connecting the circuit with which each switchis associated instantaneously to a common output line 33. Since thisprocess is no portion of the present invention and since various typeshave been shown in the copending applications already identified,specifically, in Patent No. 2,694,748 they are not here described. Theoutputs of the various circuits, combined successively by the switchingmechanism, form the reconstituted television signal, which thereafter istreated in the same manner as though picked up directly from a camerachain.

The circuit shown in Fig. 2 is a refinement of that shown in Fig. 1 andaccomplishes the same result somewhat more accurately and in a littledifferent manner. Only the reproducing portion of a single AM channel isshown, that of the other channels being mere duplication and therecording equipment being substantially identical with that alreadydescribed in connection with the first figure, so that a repetition isbelieved to be unnecessary.

In this case, the amplitude-modulated signals as picked up from the tapeare amplified and detected prior to their modulation by the lowfrequency components, the equipment for amplification and detectionbeing symbolized by the single block 37. One branch of the outputcircuit 35 leads to a low-pass filter 39. This filter, as used in aspecific apparatus here described, has a relatively sharp cut-01f; itsoutput is down only 3 db at 70 kc. but the attenuation at 100 kc. is 80db. In parallel with the filter is an overshoot suppressor 40. Theoutput of the filter 39 connects to a subtractor circuit 41, in thiscase a difierential amplifier, one form of which will be described indetail hereinafter. Here it is sufiicient to state that its efiect is tosubtract the low frequency components passed by the filter from thevideo signal as supplied from the lead 35 and feed them, via the lead43, to a conventional adding network 45.

The FM signal from one or two tracks on the tape 11, is fed to apreamplifier 15 and thence to a demodulator 16 as before. The outputfrom the demodulator passes to a low-pass filter 47, the characteristicsof which are nearly as possible identical with those of low-pass filter39. Because of these identical characteristics any phasedelays which mayoccur, or any failure of the phase delay produced to be strictlyproportional to frequency,

will be the same in the two filters. Furthermore, any ringing in thefilter output due to the sharp cut-off will also be the same in the twofilters. The output of filter 47 is fed to the adding circuit 45 withthe output of subtractor circuit, and the two signals are therecombined. Tests have indicated that the resultant signal issubstantially perfect, and that no spurious signals due to ringing,phase delays, or the like appear in the combined output circuit 49. Theeifect is a very nearly complete restoration of the low-frequencycomponents in the output signals.

The apparatus included in the subtractor 41 of Fig. 2 together with theadding circuitry, are illustrated in detail in Fig. 3. In this figurethe line 51 represents the output circuit from the FM demodulator 16.Across this line is a potentiometer 53, and the variable tap of thepotentiometer is connected to the control grid of a pentode amplifier55. The plate of the amplifier connects in parallel with the plate 56 ofone section of a twin triode 67, both of the plates being suppliedthrough a common plate resistor 59 from a common source. The cathode andscreen gn'd supplies to the tube 55 are conventional.

The other anode 56' of the twin triode 57 connects directly to 13+, thecommon source of supply. The two cathodes of the twin triode areconnected together. and to ground through series coupling resistors 61and 63.

The grid 58 of one section of the twin triode (that in which no plateresistor exists in the anode circuit) is fed with the detected output ofthe AM channel through lead 35 from the preamplifier-detector 37 of Fig.3. The signals from this circuit are also taken off from the movablecontact of a potentiometer 65, terminating the line. Through the use ofthis potentiometer a desired portion of the voltage of the signal can beselected, and this can be balanced against the low frequency voltagetaken oi the potentiometer 53 from the FM circuit. The full voltageoutput is taken off through lead 35 and supplied to the control grid ofa cathode follower tube 67 through a blocking condenser 68. Theovershoot suppressor 40, connected between condenser 68 and the grid,comprising a series resistor 69 and a damped series resonant circuit 71connecting to ground between the grid and resistor 69 may be included,but is not necessarily a part of this invention. Tube 67 is coupled bymeans of a cathode resistor 75 and a low value series resistor 77 withthe low pass filter 47, the characteristics of which have already beendescribed in connection with Fig. 2. The output of filter 47 connects toa matching impedance 79, and from the ungrounded side of the latterconnection is made to the grid 58 of the section of tube 57 whose anodeconnects in parallel with tube 55.

The operation of the circuit as as follows:

The signal from lead 35, containing both high and low frequencycomponents, is applied to the grid 58' of tube 57. This tube iscathode-follower coupled to the cathodeof the second section of thetube, passing this signal on to the cathode of the second section insubstantially a 1:1 ratio, the cathode rising in potential to almostexactly the same degree as the grid 58', following all of the frequencycomponents introduced from the amplitude-modulated tracks. At the sametime, the same frequencies are passed to cathode-follower tube 67, andfed at unity gain to filter 41, which removes the high frequencycomponents and applies the lower frequencies to grid 58 of tube 57. Asfar as the low-frequency components are concerned, this grid rises andfalls in potential concurrent- 1y with and to the same degree as thecathode, and since, for these components, there is no difference ingrid-cathode potential, the space current of the tube is not varied.There is no such concurrent variation with respect to the high frequencycomponents, and therefore the space curthus far described is rent ofthis section of the tube represents the latter components alone, down tothe frequency of cut-ofi of the filter. These frequencies are reflectedin the drop across resistor 59.

At the same time the low-frequency components, limited in frequencyrange by a filter identical with filter 41, are'applied to the grid oftube 55, varying its space current accordingly. This current, flowingthrough the common resistor 59, produces a drop therein at the lowfrequencies only, thereby restoring the DC component minus any drop-outsthat might be deflected in these components as supplied from theamplitude-modulated tracks. By adjusting the contact on potentiometer 65the portion of the voltage, delivered by the amplitude-modulated tracks,whichis supplied to the grid 58' of tube 57 can be so adjusted as tocompensate for the insertion losses introduced by the filter 47 and itsinput and output circuits, so that the low-frequency components can bebalanced out with great accuracy. Similarly, the potentiometer 53 can beso adjusted as to build up the reintroduced components to match thenormal amplitude of these components, excluding drop-outs. Output lead43 therefore carries the reestablished signal for delivery to theswitches.

Because of the methods of modulation and synchronization used underpresent standards of television transmission, it is not, in fact,necessary that the low-frequency components supplied actually includeDC. or zero frequency. The lowest frequency component which appears inthe television signal at any material amplitude is the 60 cycle fieldfrequency. When the average illumination of the field changes there is achange in the DO. component, but since the signal falls to zero levelwhen the synchronizing pulses occur the DC. level is always equal to theaverage amplitude of the A.C. components and can most readily besupplied by a DC. restorer after the signals from all channels have beencombined, in the same manner as this component is ordinarily supplied inreceivers. It is therefore satisfactory if the lowfrequency cut-off issomewhat below the 60 cycle fundamental of normal transmission.Circuitry is somewhat simplified thereby and direct-coupled amplifiersare avoided. It will be seen that A.C. coupling can be used in thesubtractor circuit, wherefrom the low-frequencies are to be removed inany event, butit is also convenient to use A.C. coupling in the outputcircuits after'the other low' frequencies have been restored.

What the visual effect of the'two forms of the invention shown will beis illustrated in Figs. 4 and 5. Fig. 4 shows an idealized signal,comprising 20 samples'or 1 complete cycle of the sampling frequency ineach of the ten'channels. For the purpose of the illustration it isassumed that the first sample taken from channel 4 is a highlight,representing a single detail of maximum illumination in the line of thepicture to be reproduced whereas the second sample taken from the samechannel is a black detail, where the signal level falls to zero. Thewhite level, the black level, and the average illumination or D.C. levelare represented by the lines so labeled in the figure. It, now, there isa drop-out in channel 4, the resultant signal, without compensation,would be that shown in Fig. by the solid lines illustrating thewaveform. The DC. level would be dropped slightly, and in place of thehigh light in the first sampling of channel 4 (designated as a in bothfigures) a black dot would be shown.

- With compensation the sample from channel No. 4 will be shown ataverage brightness. The highlight will be omitted, but so will the blackdot of the drop-out. In the second sampling, the black detail would alsobe omitted, the signal also showing the average illumination. In thisportion of the screen as a whole there will be no streak of either lightor dark. On the next scanning of that particular portion of the fieldthe probabillty is very great that no drop-out will occur, and since thescannings come so rapidly that the eye integrates the effects, andsince, moreover, small variations in contrast affect the eyesubstantially linearly Whereas at high degrees of contrast the responseapproaches the logarithmic the result of repeated scannings is that thebright portions of the picture still appear bright and the dark portionsdark. There is still a drop-out of detail but not of averageillumination, and the disturbing effect of the drop-out substantiallyvanishes.

The idealized waveform has been so drawn as to show the worst situationwith which the invention is required to cope, i.e., where the elementsdropped out are at the extremes of contrast with the average of thescene being reproduced. The greatest probability, of course, is that thedrop-outs will occur at the average value of illumination. Under thesecircumstances it would be invisible even in the absence of a precedingand succeeding frame.

The eifects produced by the two forms of the invention shown are verynearly identical. Owing to the type of filters and methods of removingand replacing D.C. level or average level of illumination, the latter isaveraged over a much shorter period in the second form shown. In theform first described the subtraction of the low-frequency components isaccomplished before the detection of the signal by a modulation process.It is because of this fact that the low-frequency components which aresubtracted can be derived from the drop-outfree signals instead of fromthe individual channel without merely subtracting and adding the samesignal and leaving the result the same as before. The signals in allchannels drop to zero level during each synchronizing pulse and whenthis occurs the wave vanishes completely: it represents 100% modulationin the negative direction. A drop-out also appears as a fall of thesignal to zero level and an attempt to subtract the drop-out-free signalcannot reduce it further. Therefore the lower frequency components aresubtracted only to the extent that they are actually present in theindividual channel affected. When the drop-out-free signals are addedthereafter they therefore raise the low-frequency components to theirproper value above zero.

In the second form of the device, where the lower fre: quencies aresubtracted after detection, the components of all frequencies aresuperimposed on the D.C. components in the space currents of the variousamplifier tubes, and to insure linearity these tubes are not carried tocut-off by zero-level signals. Therefore if the drop-outfree componentswere used for the subtraction they could reduce the signal to negativevalues, and their addition later would merely restore the originalsignal in each channel including the drop-outs. 'Therefore when post?detection subtraction is employed the subtracted signal must be derivedfrom the channel itself. The subtracted signals represent, electrically,negative illumination, which has no physical existence, but in neithercase do such signals appear in the output circuits. The low frequencycomponents are reduced to the level representative of zero illuminationand do not go negatively beyond that level. In the first instance, it isbecause negative modulation can have no more physical existence in thecircuits than can negative illumination; in the second case the negativecomponents subtracted are derived from the same signal and hence arenecessarily equal to the positive components.

While the use of frequency modulated tracks is advantageous it is notthe only means whereby a drop-out-free train of low-frequency componentscan be attained. It is possible to combine the low-frequency componentsas derived from all of the amplitude-modulated tracks and, filtering outthe high frequency components as before, use the combined signal tosupply the 'drop-out-free lowfrequencies. If these low frequencies aremerely added, a dropout in one channel will reduce their amplitudebyone-tenth, in the form of apparatus here shown. This be overcome bypicking signals from a number of 9 tracks and feeding them to acomparator circuit in which that of highest amplitude is chosen, whichavoids this difliculty.

In general, however, the use of the frequency modulated tracks ispreferred since the amplitude need not then affect the level of theproduced signals; the transducer heads, engaging two tracks, can then beconnected in series or parallel and the resultant signals amplified andthen limited so that the drop out of either channel will not afiect theover-all result.

One point of importance with regard to any of the modifications of theinvention is that while it relies upon a form of frequency division asbetween the highand low-frequency components to be displayed in thepicture, picking up the lower frequencies from certain tracks and thehigher frequencies from others, it is not subject to the diflicultieswith respect to phase-rotation and variable delay in the filters usedwhich have plagued systems using or attempting to use straight frequencydivision to accomplish the recording and reconstruction of televisionsignals. With either modification of the invention, phase rotation willtake place in the filters utilized to remove the lower frequencycomponents from the amplitude-modulated signals and to restore thosecomponents in the output. Using either type of apparatus the subtractionprocess removes the in-phase components of the signal to a greater orless extent; in the case of the simple integrating circuits of Fig. 1,at the cross-over point, where the amplitudes of the two signals areequal, one-half of the amplitude of the cross-over frequency signal issupplied from the amplitude-modulated channels and the other half fromthe frequency-modulated or other low-frequency channels. At thisfrequency there will be a 45 phase rotation in the filter. This adds anout-of-phase component in the modulation from the AM channels, butbecause of the reverse modulation it is 180 out of phase with that inthe modulating wave. It is therefore equal and opposite to the likecomponent added to the signal after the demodulation process. The sameholds true of any phase rotation that may occur near the cut-offfrequency of the filters used in Fig. 2. v

The principles involved in the various embodiments of the invention areidentical, and the difference in results is a matter of degree only.Furthermore, the various features shown may be combined in various ways,and the invention may be modified as to the method of deriving thedrop-out-free low frequencies as has already been described. The twoforms of apparatus shown and described in detail are therefore not to betaken as limiting the scope of the invention, intended limitations beingexpressed in the following claims.

What is claimed is as follows:

1. Reproducing apparatus for compensating for the efiect of drop-outs intelevision signals magnetically recorded as parallel tracks on a commonmedium, a plurality of said tracks recording modulations of carrierwaves of a common frequency phase-displaced relatively to each other toproduce substantially equal intervals between wave crests as recorded onthe various tracks and the amplitude of successively occurring crestsbeing representative of the illumination of successive elements along ascanning path of a field of view to be reproduced, which comprises: aplurality of transducer heads adapted to engage respectively each ofsaid tracks, detector means connected to. each of said transducer headsrespectively for demodulating signals reproduced thereby, means fordeveloping in a separate circuit signals representative of the lowerfrequencies only of the modulating components recorded on said tracksand means for subtracting said lower-frequency components from thedemodultated signals; means for deriving from a plurality of difierenttracks than that engaging each respective head a signal representativeof those same components subtracted from the demodulated output of suchhead, an adding circuit connected in the output of each of saiddemodulating means for adding thereto said last-men tioned signal, torestore the components removed unaffected by drop-outs in any individualtrack, and means for sampling each of the restored signals in rotationto reconstitute the recorded signal.

2. Apparatus as defined in claim 1 wherein said lowerfrequency-subtracting means comprises means for mod ulating said signalsnegatively on the signals picked up by each of said transducer headsprior to the demodulation of said signals.

3. Apparatus as defined in claim 1 wherein saidlowerfrequency-subtracting means comprises a differential amplifierresponsive only to the difierence in amplitude of the signals suppliedthereto.

4. Apparatus as defined in claim 1 wherein said different tracks arefrequency-modulated, and comprising additional transducer heads adaptedto engage said frequencymodulated track and means for demodulating thesignals developed by said additional heads to provide the lowfrequencycomponents to be added to the signals from all of the first-mentionedtracks.

5. Reproducing apparatus for compensating for the effect of drop-outs intelevision signals magnetically recorded as parallel tracks on a commonmedium, a plurality of said tracks recording modulations of carrierwaves of a common frequency phase-displaced relatively to each other toproduce substantially equal intervals between wave crests as recorded onthe various tracks and the amplitude of successively occurring crestsbeing representative of the illumination of successive elements along ascanning path of a field of view to be reproduced, which comprises: aplurality of transducer heads adapted to engage respectively each ofsaid tracks, detector means connected to each of said transducer headsrespectively for demodulating signals reproduced thereby, dividedcircuits connected to said demodulating means, a differential amplifierhaving one input terminal connected directly in one branch of each ofsaid circuits, low-pass filter means connected in the other branch ofeach of said circuits, connections from said low-pass [filters] filtermeans to another input terminal of the respective differentialamplifiers, a plurality of additional transducer heads adapted to engagetracks on said common medium, means for demodulating the signalsdeveloped by said additional heads, low-pass filter means havingsubstantially identical characteristics as said first-mentioned filtermeans connected to receive the output of said last-mentioneddemodulating means, and means for adding signals passed by saidlast-mentioned filter means to the output signals from each of saiddiflerential amplifiers.

6. Reproducing apparatus for compensating for the effect of drop-outs intelevision signals magnetically recorded as parallel tracks on a commonmedium, a plurality of said tracks recording modulations of carrierwaves of a common frequency phase-displaced relatively to each other toproduce substantially equal intervals between wave crests as recorded onthe various tracks and the amplitude of successively occurring crestsbeing representative of the illumination of successive elements along ascanning path of a field of view to be reproduced, which comprises: aplurality of transducer heads adapted to engage respectively each ofsaid tracks, detector means connected to each of said transducer headsrespectively for demodulating signals reproduced thereby, dividedcircuits connected to said demodulating means, a differential amplifierhaving one input terminal connected directly in one branch of each ofsaid circuits, low-pass filter means connected in the other branch ofeach of said circuits, connections from said low-pass filters to anotherinput terminal of the respective dilferential amplifiers, a plurality ofadditional transducer heads adapted to engage tracks on said commonmedium, frequency demodulating means con-' means having likecharacteristics to those first-mentionedv 1.1 connected for supply bysaid frequency-demodulating means, and means for adding output signalsfrom said additional filter means to signals developed by each of saiddifierential amplifiers.

7. Reproducing apparatus for compensating for the elfect of drop-outs intelevision signals magnetically recorded as parallel tracks on a commonmedium, a plurality of said tracks recording modulations of carrierwaves of a common frequency phase-displaced relatively to each other toproduce substantially equal intervals'between wave crests as recorded onthe various tracks and the amplitude of successively occurring crestsbeing representative of the illumination of successive elements along ascanning path of a field of view to be reproduced which comprises: aplurality of transducer heads adapted to engage respectively each ofsaid tracks, detector means connected to each of said transducer headsrespectively for demodulating signals reproduced thereby, dividedcircuits connected to said demodulating means, a differential amplifierhaving one input terminal connected directly in one branch of each ofsaid circuits, said differential amplifier including an output resistor,low pass filter means connected in the other branch of each of saidcircuits, connections from said low pass filter means to another inputterminal of the respective difierential amplifiers, a plurality ofadditional transducer heads adapted to engage tracks on said'commonmedium, frequency demodulating means connected to demodulate the commonoutput of said additional heads, an additional lowpass filter meanshaving like characteristics to those firstmentioned connected for supplyby said frequency-demodulating means, a plurality of amplifiersconnected for supply by said additional filter means, each having as anoutput impedance the output resistor of one of said dilferentialamplifiers, thereby to add to signals developed by said differentialamplifiers lower-frequency components from signals transduced by saidadditional heads.

8. In a system for magnetically recording and reproducing televisionSignals wherein a plurality of carrier waves, phase displacedrespectively by equal increments collectively to provide equally spacedwave crests, are modulated in rotation by samples representing theillumination of successive elements of a field of view and are recordedas parallel tracks on a common medium, the method of compensating fordrop-outs in reproduction caused by imperfections in such medium whichcomprises the steps of scanning all of said track simultaneously todevelop in separate channels wave trains representing the signals asrecorded, developing waves representative of only the lower frequenciesmodulated on the recorded waves, subtracting from the waves in eachchannel the frequencies representative of said lower frequencycomponents in the waves developed therein, simultaneously developingfrom waves in a plurality of channels waves representative of the lowerfrequency components which should be present in all channels, adding thelast mentioned waves to the waves in all channels, and sampling thewaves in the various channels in rotation to reconstruct the recordedsignals.

9. The method as defined in claim 8 wherein the subtraction and additionof said lower frequency components are accomplished by the subsidiarysteps of detecting the waves developed in said plurality of channels,filtering said waves to remove the higher-frequency componentstherefrom, inversely modulating the waves in each of said separatechannels by said filtered waves, detecting the waves in each of saidfiltered waves to the detected waves in each of said separate channelsprior to sampling them.

. 10. The method as defined in claim 8 wherein the subtraction andaddition of said lower frequency components is accomplished by thesubsidiary steps of detecting the waves in each of said separatechannels, filtering from a portion of the detected waves the higherfrequency components therein, subtracting the filtered waves fromanother portion of the detected waves in the same channel, detecting thewaves developed from said plurality of channels, filtering said lastmentioned waves to remove the higher-frequency components therefrom, andadding the filtered waves from said plurality of channels to thehigher-frequency components remaining in each of said separate channelsprior to sampling them.

11. In a reproducing system for compensating wide bandamplitude-modulated television signals for eflects caused byimperfections in a recording medium upon which the amplitude-modulatedsignals are recorded to represent information and upon whichfrequency-mod ulated signals are recorded to represent a portion of theinformation represented by the amplitude-modulated signals, first meansresponsive to the frequency-modulated signals recorded on the medium forreproducing such signals, second means responsive to theamplitude-modulated signals recorded on the medium for reproducing suchsignals, demodulating means coupled to the first means for convertingthe frequency-modulated signals from the first means into signalsmodulated in amplitude in accordance with the modulation in frequency ofthe frequency-modulated signals, and means coupled to the second meansand to the demodulating means for substituting the amplitude-modulatedsignals from the demodulating means for the corresponding portion of theamplitude-modulated signals from the second means to obtain compositesignals having information derived from both the frequency-modulatedsignals and the amplitudemodulated signals recorded on the medium.

12. The combination set forth in claim 11 in which the substitutingmeans includes means responsive to the signals from the demodulatingmeans and to the signals from the second means for obtaining thedifference between the signals from the demodulating means and thesignals from the second means to produce resultant signals and in whichthe substituting means further includes means responsive to theresultant signals from the subtracting means and to the signals from thedemodulating means for combining these signals to produce compositesignals representing the information.

13. In a recording-reproducing system for compensating for defects in arecording medium, means operatively coupled to the medium for recordingon the medium information in the form of amplitude-modulated signalswhere the amplitude modulations represent the information, meansoperatively coupled to the medium for recording on the medium at least afirst particular portion of the information in. the form offrequency-modulated signals where the frequency modulations representthe particular portion of the information, first means responsive to theamplitude-modulated signals recorded on the medium for reproducing suchsignals, second means responsive to the frequency-modulated signalsrecorded on the medium for reproducing such signals, demodulating meanscoupled to the second reproducing means for converting such signals intosignals having amplitude modulations dependent upon the frequencymodulations of the frequency-modulated signals, and means coupled to thedemodulating means and to the first reproducing means for substitutingthe amplitude-modulated signals from the demodulating means for thecorresponding portion of the amplitude-modulated signals from the firstreproducing means to obtain composite signals representing theinformation.

14. The system set forth in claim 13 in which the! substituting meansincludes means responsive to the signals from the demodulating means andfrom the first reproducing means for subtracting the signals from thedemodulating means from the signals from the first reproducing means andin which the substituting means further includes means responsive to thesignals from the demodulating means and the signals from the subtractingmeans for combining these signals to produce composite signalsrepresenting the information.

V to the signals in the first particular range of frequencies and thesignals from the first reproducing means for subtracting the signals inthe first particular range of frequencies from the signals from thefirst reproducing means to produce signals in a second particular rangeof frequencies and in which the substituting means further includesmeans responsiveto the signals in the second particular range of,frequencies and responsive to the signals from the demodulating meansfor combining such signals to produce composite signals representing theinformation.

16., In a reproducing system for compensating for the efiects ofimperfections on a surface upon which amplitttde-modulated signals andfrequency-modulated signals are recorded where the amplitude-modulatedsignals represent information and where the frequency-modulated signalsrepresent only a portion of the information represented by theamplitude-modulated signals and where the amplitude-modulated signalshave a particular range of frequencies, first means responsive to theamplitudemodulated signals recorded on the surface for producingelectrical signals having amplitude modulations corresponding to thoseof the amplitude-modulated recorded signals, second means responsive tothe frequency-modulated signals recorded on the surface for producingelectrical signals having characteristics corresponding to those of thefrequency-modulated recorded signals, frequency discriminating meanscoupled to the first means for separating the modulating components ofthe ampli tude-modulated electrical signals from the first means intosignals having a first range of frequencies and signals having a secondrange of frequencies where the second range of frequencies representsthe portion of the information corresponding to that provided by thefrequency-modulated signals and where the first and second range offrequencies constitute the particular range of frequencies, demodulatingmeans coupled to the second means for converting the frequency-modulatedsignals from the second means into signals having amplitude modulationsdependent upon the frequency modulations of the frequency-modulatedsignals, and means coupled to the separating means and to thedemodulating means for combining the signals from the demodulating meanswith the amplitude-modulated signals in the first range of frequencies.

17. In a recording-reproducing system for compensating wide bandamplitude-modulated signals having a particular range of frequencies forefiects caused by imperfections in a recording medium upon which theamplitude-modulated signals are recorded to represent information andupon which frequency-modulated signals are also recorded to represent aportion of the information represented by the amplitude-modulatedsignals, first means responsive to the frequency-modulated signalsrecorded on the recording medium for reproducing such frequencymodulatedsignals, second means responsive to the amplitude-modulated signalsrecorded on the recording medium for reproducing suchamplitude-modulated signals, means coupled to the second reproducingmeans for separating signals having a first frequency range from theamplitude-modulated signals where the first frequency stitute theparticular range of frequencies, means coupled to the first reproducingmeans for demodulating the frequency-modulated signals from the firstreproducing means into signals having amplitude modulations dependentupon the frequency modulations of the frequency-modulated signals, andmeans coupled to the demodulating means and to the subtracting means forcombining the amplitude-modulated signals from the demodulating meansand the signals in the second range of frequencies from the subtractingmeans to produce composite signals representing the information.

18. In a reproducing system for compensating for the effects ofimperfections on a surface upon which first signals are recorded with aparticular range of frequencies and with amplitude modulations torepresent information and upon which second signals are recorded torepresent a first particular portion of the information, first meansresponsive to the first signals recorded on the medium for producingsignals having amplitude modulations corresponding to the amplitudemodulations of the recorded signals, second means responsive to thesecond signals recorded on the medium for producing signals havingamplitude modulations in accordance with the particular portion of theinformation represented by such second recorded signals, amplifier meansresponsive to the amplitude-modulated signals produced by the firstmeans for passing amplitude-modulated signals in a first range offrequencies where the amplitude-modulated signals in the first range offrequencies represent a second particular portion of the informationcombining with the first particular portion of the information torepresent the information, and means responsive to theamplitude-modulated signals in the first range of frequencies from theamplifier means and the amplitudemodulated signals from the second meansfor combining such signals to produce resultant signals representing theinformation, the amplifier means including means responsive to thesignals from the first means for producing signals in a second range offrequencies where the first and second ranges of frequencies constitutethe particular range of frequencies and where the signals in the secondrange of frequencies represent the first particular portion of theinformation, the amplifier means further including means responsive tothe signals in the second range of frequencies and to the signals fromthe first means for subtracting the signals in the second range offrequencies from the signals from the first means to produce the signalsin the first range of frequencies.

References Cited in the file of this patent

